Field of the Invention
The present invention relates to a power supply, and particularly to a step-down power supply capable of changing an output voltage in a wide range and supplying power to a load.
Description of the Related Art
In general, a fan used in an electronic device has, in terms of noise, the function of changing the rotation speed thereof in accordance with an operation mode of the device. Control is performed such that the rotation speed of the fan is increased and cooling of the inside of the device is prioritized when the device is being operated, and the rotation speed of the fan is reduced to reduce noise when the device is on standby. In general, the rotation speed of a fan changes in accordance with a supplied voltage. Thus, as a power supply that drives a fan, a step-down power supply is used that is capable of changing an output voltage in a wide range (see Japanese Patent Laid-Open No. 2007-116804).
In the case where an output voltage is not changed, a power supply circuit in Japanese Patent Laid-Open No. 2007-116804 performs a switching operation in a frequency range in which certain power supply efficiency is achieved by feedback control. In contrast, in the case where the output voltage is changed, a switching operation is performed using a switching-frequency control table based on setting voltages. Here, it is known that in the case where the output voltage is changed in a wide range with a less expensive configuration and where a load does not require high voltage accuracy, control is performed using a switching control table based on setting voltages, without performing feedback control.
In the following, a conventional step-down power supply circuit described above will be described with reference to FIG. 5A. This power supply circuit is comprised of a switch circuit and an output circuit. The switch circuit switches on and off switching elements Q1 and Q2 using an FPWM signal. A direct input voltage Vin input to an input terminal is then converted into a periodic pulsed-wave signal Vpulse. This signal is smoothed by an output circuit comprised of a diode D2, an inductor L1, and a capacitor C1, and is output to an output terminal as an output voltage Vout. In this circuit, without performing feedback control, an output voltage is changed by changing the pulse width of the FPWM signal.
Here, a circuit as illustrated in FIG. 5B and in which a resistor R3 is used instead of the diode D2 and the inductor L1 in the output circuit has conventionally been used. In this circuit, the periodic pulsed-wave signal Vpulse output from the switch circuit is smoothed by the resistor R3 and a capacitor C1, and is output as an output voltage Vout. Similarly to as in the circuit of FIG. 5A, an output voltage is also able to be changed by changing the pulse width of an FPWM signal in this circuit.
In addition, in the case where an output voltage is adjusted to a constant value and where a load requires high voltage accuracy, a circuit as illustrated in FIG. 5C has conventionally been used. In this circuit, the output voltage is able to be switched between two types of voltage: a voltage close to the level of the input voltage (hereinafter referred to as input-voltage level), and a voltage lower than the input-voltage level. In the case where the voltage close to the input-voltage level is output, the voltage is output by setting an F_ON signal to High and switching on switching elements Q3 and Q4. In the case where the voltage lower than the input-voltage level is output, the voltage is output with high voltage accuracy by setting an F_H_ON signal to High, switching on switching elements Q1 and Q2, and conducting a Zener diode ZD2. Regarding paths for these two output voltages, a control signal and a control signal line are necessary for each of the paths in this circuit.
However, the inductor L1 used in the conventional art illustrated in FIG. 5A is arranged on a route through which power is supplied to a load, and thus a large part needs to be used, thereby increasing the circuit in size. In contrast, in the conventional art illustrated in FIG. 5B and that does not use the inductor L1, it is difficult to output a desired voltage because of a voltage drop in the resistor R3 in the case where the output voltage close to the input-voltage level is requested. It is possible to output a voltage close to the input-voltage level by increasing the number of control signal lines and the number of dedicated output lines as in the circuit illustrated in FIG. 5C; however, there is a problem in that when the number of output control signals is increased, the number of necessary CPU pins is increased.